Illegal immigration across the southwestern border is down a stunning 76 percent since President Trump was elected, with the flow of children and families dropping even faster as analysts say the administration’s commitment to enforcing the law has changed the reality along the border.

Overall apprehensions by the Border Patrol dropped to just 11,129 in April, according to numbers released Tuesday, marking the lowest monthly total for any month in decades.

The number of unaccompanied illegal immigrant children nabbed at the border dropped below 1,000 — a level not seen since before the surge that bedeviled President Obama during most of his second term.

Even before a foot of Mr. Trump’s planned border wall is built or any more agents are hired, the threat of being sent home has forced would-be migrants to rethink making the journey, officials said.

“A lot of the discussion about changes in our enforcement policy and the way we are going about doing business, we believe that has deterred people,” said Homeland Security spokesman David Lapan. “When you get here, it is likely you are going to get caught. You are going to be returned to your country.”

That approach marks a major change from the Obama administration, which struggled to handle the flow of illegal immigrants from Central America.

Under Obama-era policies, hundreds of thousands of children and families from El Salvador, Guatemala and Honduras were caught and then released into the interior of the U.S., where they often failed to show up for their deportations and instead disappeared into the shadows.

Mr. Trump has vowed quick deportations and has called for expanding detention facilities to hold illegal immigrants in the meantime, preventing them from slipping away.

“This is messaging, backed up by actual enforcement and policy changes that people are responding to,” said Jessica Vaughan, policy studies director at the Center for Immigration Studies. “The continued drop suggests this is more than just a fluke.”

Border apprehensions are considered a rough yardstick for the overall flow of illegal immigration, so a drop in arrests is believed to reflect an overall drop in the flow of people.

At its peak early in the last decade, the Tucson sector, which is just one of nine regions along the border, regularly recorded more than 70,000 apprehensions in a single month. Last month, Tucson reported fewer than 1,500 arrests.

Activity has shifted to the Rio Grande Valley sector in Texas, with nearly 4,000 apprehensions in April. Still, that’s a fraction of the 22,000 apprehensions recorded in October and fewer still than the nearly 40,000 arrests at the peak of the Central American surge in 2014.

Authorities expect a seasonal uptick in border apprehensions this month and next but are waiting to see the degree of any seasonal surge.
One indication that the change is a result of immigration enforcement rather than better border security is the flow of drugs, which remains high.

Mr. Lapan said that while seizures of marijuana are down, hard drugs including heroin, cocaine and methamphetamine are up. Like migration, a rise in seizures is believed to signal an increase in the overall flow.
“We are still seeing a lot of illicit drugs come into the country,” Mr. Lapan said.

As attention at the border has increased, migrants during the later part of the Obama administration increasingly decided to show up at official border crossings, demanding entry into the U.S.

Some of them would lodge asylum claims, having been coached by family and friends of activists on “magic words” that would earn them tentative legal status in the U.S., the former Border Patrol chief testified to Congress last year.

Those numbers have also dropped dramatically, from about 15,000 a month late last year to fewer than 5,000 a month under Mr. Trump.
Haitians in particular had been abusing that system, with nearly 3,500 showing up on the doorstep of the U.S. in October. But that number fell to just 49 in April.

Immigrant rights groups argue that many of the Central Americans should be treated not as illegal immigrants but as refugees deserving of protection.
In the Tuesday report, the American Immigration Council called for the government to stop putting children and families into detention and said border agents and officers are preventing deserving migrants from lodging their claims.

“The protection needs of asylum seekers, and asylum-seeking mothers and their children in particular, must be met with robust legal services and legal assistance from the start to ensure that no one is sent back to their deaths,” the council said.

But even Obama administration officials have said using detention and quick deportations works in stemming a flow of illegal immigrants.

“If you came across after paying $6,000, which is pretty tough to come by in those countries, and you were detained and then sent home, back to your village, it sends a pretty powerful message and, frankly, has a chilling effect on people,” R. Gil Kerlikowske, the former head of U.S. Customs and Border Protection, said at a forum this week hosted by the Migration Policy Institute.